Colonization of the Galpagos Islands by Plants with No Specific
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Ecography 35: 33–43, 2012 doi: 10.1111/j.1600-0587.2011.06980.x © 2011 Th e Authors. Ecography © 2012 Nordic Society Oikos Subject Editor: Francisco Pugnaire. Accepted 5 April 2011 Colonization of the Gal á pagos Islands by plants with no specifi c syndromes for long-distance dispersal: a new perspective Pablo Vargas , R. Heleno , A. Traveset and M. Nogales P. Vargas ([email protected]), Real Jard í n Bot á nico de Madrid (CSIC-RJB), ES-28014 Madrid, Spain. – R. Heleno, Charles Darwin Founda- tion, Puerto Ayora, Santa Cruz, Gal á pagos, EC-17-1-3891 Quito, Ecuador. RH also at: Inst. Mediterrani d ’ Estudis Avan ç ats (CSIC-UIB), ES-07190 Esporles, Mallorca, Balearic Islands, Spain. – A. Traveset, Inst. Mediterrani d ’ Estudis Avan ç ats (CSIC-UIB), ES-07190 Esporles, Mallorca, Balearic Islands, Spain. – M. Nogales, Island Ecology and Evolution Research Group (CSIC-IPNA), ES-38206 La Laguna, Tenerife, Canary Islands, Spain. Since nobody has witnessed the arrival of early plant colonists on isolated islands, the actual long-distance dispersal (here- after LDD) has historically been a matter of speculation. In the present study, we off er a new approach that evaluates whether particular syndromes for LDD (i.e. the set of traits related to diaspore dispersal by animals, wind and sea cur- rents) have been favourable in the natural colonization of the Gal á pagos Islands by plants. Dispersal syndromes of the 251 native genera (509 angiosperm species) presently acknowledged as native were carefully studied, combining data from fl oristic lists of the Gal á pagos Islands, diaspore traits, characteristics of continental relatives and our own observations. We used these genera (and occasionally infrageneric groups) as the working units to infer the number of introductions and colonists. A fi nal number of native plants was inferred and analysed after correcting by pollen records of six species from six genera previously considered exotic (palaeobotanical correction). Th e number of early colonists was also corrected by incorporating information from the few ( n ϭ 12) phylogenetic studies of genera from both the Gal á pagos Islands and the Americas (phylogenetic correction). A total of 372 colonization events were inferred for the native fl ora using the latest check-list. Th e proportions of native colonists grouped into fi ve categories were: endozoochory 16.4%, epizoochory 15.7%, hydrochory 18.6%, anemochory 13.3%, and unassisted diaspores 36.0%. Th ese fi gures did not vary signifi cantly on analysing only the 99 genera that include endemic species in order to rule out any human-mediated introductions. Irrespective of the roles of the diff erent agents involved in LDD, diaspores with no special syndrome for LDD (unas- sisted diapores), such as many dry fruits, have been successful in reaching and colonizing the Gal á pagos archipelago. Th is fi nding leads us to suggest that unpredictable and so far unknown LDD mechanisms should be further considered in the theory of island biogeography. Islands are ideal models to study long-distance plant dis- of reconstructing the colonization history of each archi- persal (hereafter LDD) (Gillespie and Clague 2009). Seed pelago has led scholars to infer, and often speculate, about dispersal is often the fi rst step for plants to colonize new actual vectors responsible for the presence of plant groups territories and thus is crucial to understanding species com- on remote oceanic islands. Wagner et al. (1990) calculated position, range expansion and genetic structure (Cain et al. for the Hawaiian fl ora that the ca 1000 native species arose 2000, Nathan 2006). Traditionally, taxonomic and fl oristic from 270–280 colonists. According to Carlquist (1967), data have provided the basic tool to infer the geographic Hawaiian colonists were dispersed by birds (73%), oceanic origin of fl owering plants on oceanic islands (Hooker 1847, drift (23%) and wind (4%). For the Gal á pagos fl ora (436 Carlquist 1967, MacArthur and Wilson 1967). Given their native species), Porter (1983) also suggested that a large relatively recent origin from the sea-fl oor in comparison to proportion (60%) of all fl owering plants had been intro- continental areas, researchers have focused on the bioge- duced by birds, a lower fraction by wind (31%), and the ography of LDD events in oceanic islands. Th e number of rest by ocean drift (9%). taxa (mainly genera) has been widely used to estimate the Which dispersal agent was responsible for the presence number of colonization events. In addition, description of of a plant group in an archipelago is a matter of specu- the vegetative and diaspore structures of plants has aided lation, because fruits and seeds may have arrived on an in categorising them into four main groups of diaspores island by unpredictable means (Higgins et al. 2003). Rather related to dispersal by: wind (anemochory), water (hydro- than speculating on the colonization vector, a more test- chory), internally by animals (endozoochory), and exter- able exercise would be to evaluate the relative presence of nally by animals (epizoochory) (Carlquist 1967). Th e lure diff erent plant dispersal syndromes occurring on islands, 33 regardless of how the fi rst propagules actually arrived Materials and methods (Vargas 2007). Previous authors, however, claimed for a high number of seeds accidentally introduced by mud Number of colonization events adherence (41 colonists) for the Gal á pagos Islands (Porter 1983), even if there is no solid evidence for such events. A full list of native plant genera of angiosperms from the Direct observations of the arrival of new diaspores Gal á pagos Islands was taken from an annotated checklist are unlikely and inferences of means of dispersal used by of angiosperms by Lawesson et al. (1987). An additional early founders cannot be reliably tested. In contrast, the fl oristic list (Porter 1983) was also analysed for comparison. contribution of various dispersal syndromes of plants that Floristic data were systematically evaluated along the fol- have colonized remote archipelagos (like the Gal á pagos) lowing complementary steps: 1) we used genera as opera- can be estimated. tional starting units (Table 1); 2) only native taxa indicated In order to correctly assess whether the four LDD syn- in fl oristic studies were considered; 3) the initial list of the dromes have been favourable in the natural colonization of native species was amended according to recent palaeobo- oceanic islands, it is necessary to control for some factors tanical evidence (palaeobotanical correction); 4) more than obscuring correct inferences. Th e most confounding factor one ancestor was adopted for genera containing multiple is colonization by plants brought by humans and therefore taxonomic groups (subgenera, sections, subsections); and 5) not attributable to natural dispersal. Th e Gal á pagos have the inferred number of original colonists was corrected by been continuously colonized since their formation (suba- phylogenetic evidence (phylogenetic correction), in which erial volcanic rocks of Ͻ 6 Ma, Geist 1996). Th is period is each independent, monophyletic group accounts for a single considered long enough for the islands to receive natural origin (Table 2). Fossil records and phylogenetic reconstruc- introductions that may have diff ered into morphologi- tions (including phylogeography) are two powerful tools to cal varieties (endemic taxa). Th us, the endemic (and then inform hypotheses of island colonization. We used published native) status of these taxa is simply identifi ed using mor- fi ndings of fossil pollen and plant remains preserved in sedi- phological characters. For non-endemic taxa, natural vs mentary deposits to adjust previous assumptions of native- human-mediated introductions are often diffi cult to tell ness of the Gal á pagos plants (van Leeuwen et al. 2008). We apart (Tye 2006). Nevertheless, pollen records predating assume that every single Gal á pagos lineage originates from the fi rst human arrival (1535) can help determine natural a mainland lineage. A minimal number of introductions introductions (van Leeuwen et al. 2008). can, however, be inferred from phylogenetic and phylogeo- Th e appropriate unit to test the success of LDD syn- graphic topologies as long as a signifi cant sample from the dromes is the number of colonizations itself, which is Gal á pagos and the mainland is provided: unknown. Initial working units for the number of intro- 1) a monophyletic group only involving island species of ductions are genera, assuming an independent origin for the same plant group indicates a single colonization; each. However, a single genus can be the result of more 2) two or more independent clades of the same genus than one introduction, particularly large genera including (i.e. unrelated species) are interpreted as the arrival of infrageneric taxa. Th erefore, multiple dispersal events from two or more colonists from diff erent mainland lineages; the same plant group (genus, subgenus, species, lineage) 3) two or more independent clades of the same species may greatly increase the number of successful dispersal (i.e. unrelated individuals) are interpreted as the arrival of events undetected by morphology. In addition to mor- two or more colonists from diff erent mainland lineages. phological inferences, phylogenetic and phylogeographic Accordingly, the number of inferred colonizations was methods are essential tools at the species and population re-assessed by considering the most recent common ances- levels, inasmuch as they provide precise reconstructions of tors of each Gal á pagos and mainland groups, for which lineage relationships for Gal á pagos and continental plants clade topologies were used to correct the number of colo- (Andrus et al. 2009). In particular, phylogenetic methods nists (Vargas 2007). aid in evaluating the number of colonization events (test- We followed previous approaches as much as possible ing monophyletic groups), geographical sources of origin in testing the number of native species and number of (inferring sister-group relationships) and shifts of ancestral natural introductions (Porter 1983, Lawesson et al.